1. Field of the Invention
The present invention relates generally to adhesively applied external prosthetic materials for the human body and, more particularly, to improved materials for use in maxillofacial reconstructive prostheses for application to correct facial defects. The materials are characterized by improved adherence, tear strength and color retention qualities.
2. Related Art
Maxillofacial prosthetic materials are materials externally applied to correct facial defects resulting from cancer surgery, accidents, congenital deformities or the like. The major requirements for the materials include high resiliency, high tear resistance and tensile strength, adequate softness, miscibility with the colorants, compatibility with adhesive material and facial tissues, and the materials must be non-toxic and non-irritating.
Several plastic materials, including acrylic polymers, silicones and polyurethanes, have been used for maxillofacial reconstruction. Among them, polysiloxanes, or the so-called silicones, have been used most widely. This is mainly due to several generally favorable qualities including ease of fabrication, high degree of chemical inertness, low degree of toxicity, and high degree of thermal and oxidative stability. The uniqueness of polysiloxanes is that siloxane bonds Si--O--Si in the main chains, as well as Si--C bonds where side groups are bonded to silicon, are extremely flexible and have a great freedom of motion. This is reflected in their lower viscosity, lower surface tension, lower melting point and glass transition temperatures, and is responsible for the elastomeric behaviors of many polysiloxanes.
Some discussion of the chemistry of silicone elastomers should be helpful in understanding the invention. In general, to be useful polysiloxanes must be cross-linked to form solid elastomer materials. Cross-linking of polysiloxanes can be achieved by any of the following process mechanisms:
1. Free radical cross-linking of linear polysiloxanes through the use of organic peroxides, e.g. benzoyl peroxide, at elevated temperature. The method is applicable to both polysiloxanes with unreactive end groups and polysiloxanes with reactive group, e.g. vinyl groups. The resulting materials are known as heat-vulcanized (HTV) silicones, PA1 2. Cross-linking of linear polysiloxane with reactive end groups such as silanols (hydroxyl-terminated polysiloxanes). This method of crosslinking requires a cross-linking agent, e.g. tetraethyl silicate, and a catalyst, e.g. dibutyl tin dilaurate, and is a condensation reaction by nature. PA1 3. Cross-linking of polysiloxanes by addition reactions. The reactions involve the addition of silyl hydride groups (--SiH) to vinyl groups (CH.sub.2 .dbd.CH--) attached to silicon with the aid of a platinum containing catalyst. PA1 where R is H or CH.sub.3, have been proposed for an internal use, namely, as the primary constituent material for a soft denture liner as disclosed in U.S. Pat. No. 5,268,396 issued to Juey H. Lai, the inventor in the present application. That document is deemed incorporated herein by reference for any and all purposes. PA1 where R is H or CH.sub.3.
The silicone elastomers themselves, including those that have been used for maxillofacial reconstruction, are generally classified into two types according to the crosslinking process, namely, heat-vulcanized (HTV) silicones and room temperature-vulcanized (RTV) silicones.
As indicated above, HTV silicones are polydimethylvinylsiloxane copolymers which have at least a small percentage of vinyl groups in the side chains of the polymers. The copolymer undergoes crosslinking according to the mechanism 1 as described above. Crosslinking occurs at the vinyl groups at high temperature (&gt;100.degree. C.) with the aid of a thermal initiator, e.g. benzoyl peroxide, which acts as the free-radical polymerization and crosslinking initiator. Heat decomposes the initiator into free radicals which initiate the crosslinking of the polymer molecules and convert the linear polymer into a resilient elastomer. The heat activated crosslinking reaction is in the nature of an additional reaction which does not produce any by-product.
Two types of RTV silicones have been used. The first type is produced by the crosslinking of hydroxyl-terminated polydimethylsiloxanes (I) as in mechanism 2 above. ##STR1## where n is an integer having a value from 1 to about 10,000.
The crosslinking is a condensation reaction by nature, and requires a crosslinking agent. In Medical Adhesive Type A (Dow Corning), methyl triacetoxy silane (II) is used as the crosslinking agent. ##STR2##
The crosslinking, however, requires water molecules to hydrolyze the silane and as seen in (II), produces acetic acid (an irritant) as the by-product. Since Medical Adhesive Type A requires moisture to cure at room temperature, the cure time is excessively long making it impractical for curing the material inside a mold. The use of Medical Adhesive Type A has therefore been limited to that of an extrinsic colorant carrier applied to the surface of the prosthesis. Further, the siloxane bonds formed by the condensation reaction are susceptible to degradation reactions such as hydrolysis. The prostheses have relatively low tear strength and are incapable of maintaining edge resistance.
The second type of RTV silicone involves the crosslinking of polysiloxanes by addition reactions. The reactions generally involve the addition of silyl hydride groups (--SiH) to vinyl groups (CH.sub.2 .dbd.CH--) attached to the silicone with the aid of a platinum containing catalyst (III) as in mechanism 3, above. ##STR3## where R, R', R" and R"' groups are alkyl groups (almost exclusively methyl groups).
The second type of RTV silicones are not truly RTV silicones. The curing of these silicones, in fact, requires heating the material at 150.degree. C. for a time, possibly an hour. Although these materials have improved tear strength over the first type of RTV silicones, they are also very hydrophobic. Generally, these materials have low adhesion to non-silicone-based adhesives, and do not readily accept extrinsic coloration. Further, the cure of the materials may be inhibited by traces of amines, sulfur, nitrogen oxides and organo-tin compounds.
The current siloxane maxillofacial materials are generally RTV silicones which are supplied as two parts. As soon as the catalyst and the crosslinker are mixed, the curing reactions start immediately. Therefore, the working time for these materials is rather limited. This is especially troublesome when de-airing of the mixture requires more than 30 minutes of vacuum pumping. In addition, the catalysts have a rather short shelf life (6 months). Furthermore, since the cure of the materials can be inhibited by traces of various impurities, use of pigments and colorants free of the impurities is required to prevent the cure inhibition. This severely limits the choice of colorants.
Polysiloxane materials including acryloxy- and methacryloxyalkyl-terminated polydimethylsiloxanes (MPDS) of the following general schematic structural formula: ##STR4## where m is an integer having a value from 1 to about 6, where n is an integer having a value from 1 to about 10,000, and
Thus, although a number of polysiloxane-based maxillofacial prosthetic materials are currently available and are at least partially successful, current polysiloxane-based maxillofacial prosthetic materials are still far from ideal. Major problems associated with the current prosthetic materials include degradation of physical properties, discoloration of the prostheses, and the difficulty of repairing.
Despite the progress that has been made in the area of maxillofacial prostheses, there remains a need to provide a siloxane maxillofacial material of the low temperature HTV type that exhibit or retains high tear strength but which readily accepts extrinsic coloration, is less hydrophobic and which, once formulated and mixed, has an increased working or open time and a long shelf life.
Accordingly, it is a primary object of the invention to provide an improved maxillofacial prosthetic material having improved properties.
Another object of the invention is to provide a maxillofacial prosthetic material having improved facial adherence properties.
Yet another object of the invention is to provide a maxillofacial prosthetic material having improved physical characteristics.
A further object of the present invention is to provide a maxillofacial prosthetic material having improved color retention qualities.
A still further object of the invention is to provide a maxillofacial prosthetic material of the low temperature HTV silicone type which afford an indefinite open or working time prior to cure.
A yet still further object of the present invention is to provide a maxillofacial prosthetic material having an extended shelf life.
Other objects and advantages of the invention will become apparent to those skilled in the art upon familiarization with the specification including the examples in the claims of this application.